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href="/fe/javascript/debounce_throttle.html" class="sidebar-link">防抖和节流</a></li><li><a href="/fe/javascript/visible.html" class="sidebar-link">面试官：如何判断一个元素是否在可视区域中？</a></li><li><a href="/fe/javascript/continue_to_upload.html" class="sidebar-link">面试官：大文件上传如何做断点续传？</a></li><li><a href="/fe/javascript/pull_up_loading_pull_down_refresh.html" class="sidebar-link">面试官：如何实现上拉加载，下拉刷新？</a></li><li><a href="/fe/javascript/single_sign.html" class="sidebar-link">面试官：什么是单点登录？如何实现？</a></li><li><a href="/fe/javascript/var_let_const.html" class="sidebar-link">var 、let 和 const 区别？实现原理是什么？</a></li><li><a href="/fe/javascript/null_undefined.html" class="sidebar-link">undefined与null的区别</a></li><li><a href="/fe/javascript/data_type.html" class="sidebar-link">数据类型</a></li><li><a href="/fe/javascript/js.html" class="sidebar-link">JavaScript</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>TypeScript</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ES6</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue3</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>React</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Webpack</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Node.js</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="this"><a href="#this" class="header-anchor">#</a> this</h1> <p><img src="/assets/img/this.00c4cddb.png" alt="this"></p> <h3 id="_7-this"><a href="#_7-this" class="header-anchor">#</a> 7. this</h3> <ul><li><a href="https://github.com/axuebin/articles/issues/6" target="_blank" rel="noopener noreferrer">JavaScript基础心法——this<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul> <p>概念：在JavaScript中，this的指向是调用时决定的，而不是创建时决定的</p> <h4 id="_7-1-this指向"><a href="#_7-1-this指向" class="header-anchor">#</a> 7.1 this指向</h4> <ul><li>全局上下文：window对象</li> <li>函数上下文：
<ul><li>直接调用：<code>this</code>指向全局变量，即 <code>window</code>。</li> <li>call()、apply()：<code>this</code>指向绑定的对象上。</li> <li>bind()：<code>this</code>将永久地被绑定到了<code>bind</code>的第一个参数。</li> <li>箭头函数：箭头函数会捕获其所在上下文的this值，作为自己的<code>this</code>值。</li> <li>作为对象的一个方法：<code>this</code>指向调用函数的对象。</li> <li>作为一个构造函数：<code>this</code>被绑定到正在构造的新对象。</li> <li>作为一个<code>DOM</code>事件处理函数：<code>this</code>指向触发事件的元素，也就是始事件处理程序所绑定到的<code>DOM</code>节点。</li> <li>HTML标签内联事件处理函数：<code>this</code>指向所在的DOM元素</li> <li>jQuery的this：在许多情况下<code>JQuery</code>的<code>this</code>都指向<code>DOM</code>元素节点。</li></ul></li></ul> <h4 id="_7-2-call-apply-bind"><a href="#_7-2-call-apply-bind" class="header-anchor">#</a> 7.2 call/apply/bind</h4> <p>通过call、apply、bind来将this绑定到指定的对象上。三者都是用来改变函数的this指向。</p> <p>注意：第一个参数是<code>undefined</code>或<code>null</code>时它会被替换成全局对象</p> <p><code>call</code>与<code>applay</code>唯一区别：后面的参数，其中<code>call</code>以一个一个的形式传递，<code>apply</code>以数组的形式传递。</p> <p><code>bind</code>: bind() 方法会创建一个新函数（称之为绑定函数），原函数的一个拷贝，也就是说不会像call和apply那样立即执行。</p> <h2 id="一、定义"><a href="#一、定义" class="header-anchor">#</a> 一、定义</h2> <p><code>this</code> 关键字是函数运行时自动生成的一个内部对象，只能在函数内部使用，总指向调用它的对象。</p> <p>函数的 <code>this</code> 关键字在 <code>JavaScript</code> 中的表现略有不同，此外，在严格模式和非严格模式之间也会有一些差别。</p> <p>在绝大多数情况下，函数的调用方式决定了 <code>this</code> 的值（运行时绑定）</p> <p>举个例子：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 当前调用栈是：baz</span>
    <span class="token comment">// 因此，当前调用位置是全局作用域</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> <span class="token string">&quot;baz&quot;</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &lt;-- bar的调用位置</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 当前调用栈是：baz --&gt; bar</span>
    <span class="token comment">// 因此，当前调用位置在baz中</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> <span class="token string">&quot;bar&quot;</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &lt;-- foo的调用位置</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 当前调用栈是：baz --&gt; bar --&gt; foo</span>
    <span class="token comment">// 因此，当前调用位置在bar中</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> <span class="token string">&quot;foo&quot;</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &lt;-- baz的调用位置</span>
</code></pre></div><p>同时，<code>this</code> 在函数执行过程中，<code>this</code> 一旦被确定了，就不可以再更改</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  a<span class="token operator">:</span> <span class="token number">20</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">this</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span> <span class="token comment">// 修改this，运行后会报错</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><h2 id="二、绑定规则"><a href="#二、绑定规则" class="header-anchor">#</a> 二、绑定规则</h2> <p>根据不同的使用场合，<code>this</code> 有不同的值，主要分为下面几种情况：</p> <ul><li><p>默认绑定</p></li> <li><p>隐式绑定</p></li> <li><p>new绑定</p></li> <li><p>显式绑定</p></li></ul> <h3 id="_2-1-默认绑定"><a href="#_2-1-默认绑定" class="header-anchor">#</a> 2.1 默认绑定</h3> <p>全局环境中定义 <code>person</code> 函数，内部使用 <code>this</code> 关键字</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> name <span class="token operator">=</span> <span class="token string">'Jenny'</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">person</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">person</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Jenny</span>
</code></pre></div><p>上述代码输出 <code>Jenny</code>，原因是调用函数的对象在游览器中位 <code>window</code>，因此 <code>this</code> 指向 <code>window</code>，所以输出 <code>Jenny</code>。</p> <p><strong>注意</strong>：严格模式下，不能将全局对象用于默认绑定，<code>this</code> 会绑定到 <code>undefined</code>，只有函数运行在非严格模式下，默认绑定才能绑定到全局对象</p> <h3 id="_2-2-隐式绑定"><a href="#_2-2-隐式绑定" class="header-anchor">#</a> 2.2 隐式绑定</h3> <p>函数还可以作为某个对象的方法调用，这时 <code>this</code> 就指这个上级对象</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>m <span class="token operator">=</span> test<span class="token punctuation">;</span>

obj<span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1</span>
</code></pre></div><p>这个函数中包含多个对象，尽管这个函数是被最外层的对象所调用，<code>this</code> 指向的也只是它上一级的对象</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token punctuation">{</span>
    a<span class="token operator">:</span> <span class="token number">10</span><span class="token punctuation">,</span>
    b<span class="token operator">:</span> <span class="token punctuation">{</span>
        <span class="token function-variable function">fn</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
o<span class="token punctuation">.</span>b<span class="token punctuation">.</span><span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>上述代码中，<code>this</code> 的上一级对象为<code>b</code>，<code>b</code>内部并没有<code>a</code>变量的定义，所以输出 <code>undefined</code></p> <p>这里再举一种特殊情况</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token punctuation">{</span>
    a<span class="token operator">:</span> <span class="token number">10</span><span class="token punctuation">,</span>
    b<span class="token operator">:</span> <span class="token punctuation">{</span>
        a<span class="token operator">:</span><span class="token number">12</span><span class="token punctuation">,</span>
        <span class="token function-variable function">fn</span><span class="token operator">:</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
            console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// window</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> j <span class="token operator">=</span> o<span class="token punctuation">.</span>b<span class="token punctuation">.</span>fn<span class="token punctuation">;</span>
<span class="token function">j</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>此时 <code>this</code> 指向的是 <code>window</code>，这里的大家需要记住，<code>this</code> 永远指向的是最后调用它的对象，虽然<code>fn</code>是对象<code>b</code>的方法，但是<code>fn</code>赋值给<code>j</code>时候并没有执行，所以最终指向 <code>window</code>。</p> <h3 id="_2-3-new绑定"><a href="#_2-3-new绑定" class="header-anchor">#</a> 2.3 new绑定</h3> <p>通过构建函数<code>new</code>关键字生成一个实例对象，此时<code>this</code>指向这个实例对象</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
　<span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>x <span class="token comment">// 1</span>
</code></pre></div><p>上述代码之所以能过输出1，是因为<code>new</code>关键字改变了<code>this</code>的指向。</p> <p>这里再列举一些特殊情况：</p> <p><code>new</code>过程遇到<code>return</code>一个对象，此时<code>this</code>指向为返回的对象。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>user <span class="token operator">=</span> <span class="token string">'xxx'</span><span class="token punctuation">;</span>  
    <span class="token keyword">return</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>  
<span class="token punctuation">}</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">.</span>user<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
</code></pre></div><p>如果返回一个简单类型的时候，则 <code>this</code> 指向实例对象。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>user <span class="token operator">=</span> <span class="token string">'xxx'</span><span class="token punctuation">;</span>  
    <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">fn</span><span class="token punctuation">;</span>  
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">.</span>user<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// xxx</span>
</code></pre></div><p>注意的是<code>null</code>虽然也是对象，但是此时<code>new</code>仍然指向实例对象。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>user <span class="token operator">=</span> <span class="token string">'xxx'</span><span class="token punctuation">;</span>  
    <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">fn</span><span class="token punctuation">;</span>  
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">.</span>user<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// xxx</span>
</code></pre></div><h3 id="_2-4-显式修改"><a href="#_2-4-显式修改" class="header-anchor">#</a> 2.4 显式修改</h3> <p><code>apply()、call()、bind()</code>是函数的一个方法，作用是改变函数的调用对象。它的第一个参数就表示改变后的调用这个函数的对象。因此，这时<code>this</code>指的就是这第一个参数</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
　console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>m <span class="token operator">=</span> test<span class="token punctuation">;</span>
obj<span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// 1</span>
</code></pre></div><p><a href="/fe/javascript/bind_call_apply.html">apply、call、bind三者的区别</a></p> <h2 id="三、箭头函数"><a href="#三、箭头函数" class="header-anchor">#</a> 三、箭头函数</h2> <p>在 ES6 的语法中还提供了箭头函数，让我们在代码书写时就能确定 <code>this</code> 的指向（编译时绑定）</p> <p>举个例子：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">sayThis</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

obj<span class="token punctuation">.</span><span class="token function">sayThis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// window 因为 JavaScript 没有块作用域，所以在定义 sayThis 的时候，里面的 this 就绑到 window 上去了</span>
<span class="token keyword">const</span> globalSay <span class="token operator">=</span> obj<span class="token punctuation">.</span>sayThis<span class="token punctuation">;</span>
<span class="token function">globalSay</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// window 浏览器中的 global 对象</span>
</code></pre></div><p>虽然箭头函数的<code>this</code>能够在编译的时候就确定了<code>this</code>的指向，但也需要注意一些潜在的坑</p> <p>下面举个例子：</p> <p>绑定事件监听</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> button <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'mngb'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
button<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'click'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">===</span> window<span class="token punctuation">)</span> <span class="token comment">// true</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>innerHTML <span class="token operator">=</span> <span class="token string">'clicked button'</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>上述可以看到，我们其实是想要<code>this</code>为点击的<code>button</code>，但此时<code>this</code>指向了<code>window</code></p> <p>包括在原型上添加方法时候，此时<code>this</code>指向<code>window</code></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token class-name">Cat</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">sayName</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">===</span> window<span class="token punctuation">)</span> <span class="token comment">//true</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name
<span class="token punctuation">}</span>
<span class="token keyword">const</span> cat <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Cat</span><span class="token punctuation">(</span><span class="token string">'mm'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
cat<span class="token punctuation">.</span><span class="token function">sayName</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre></div><p>同样的，箭头函数不能作为构建函数</p> <h2 id="四、优先级"><a href="#四、优先级" class="header-anchor">#</a> 四、优先级</h2> <h3 id="隐式绑定-vs-显式绑定"><a href="#隐式绑定-vs-显式绑定" class="header-anchor">#</a> 隐式绑定 VS 显式绑定</h3> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">var</span> obj1 <span class="token operator">=</span> <span class="token punctuation">{</span>
    a<span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
    foo<span class="token operator">:</span> foo
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">var</span> obj2 <span class="token operator">=</span> <span class="token punctuation">{</span>
    a<span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span>
    foo<span class="token operator">:</span> foo
<span class="token punctuation">}</span><span class="token punctuation">;</span>

obj1<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>
obj2<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>

obj1<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span> obj2 <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
obj2<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span> obj1 <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>
</code></pre></div><p>显然，显示绑定的优先级更高</p> <h3 id="new绑定-vs-隐式绑定"><a href="#new绑定-vs-隐式绑定" class="header-anchor">#</a> new绑定 VS 隐式绑定</h3> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">something</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> something<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> obj1 <span class="token operator">=</span> <span class="token punctuation">{</span>
    foo<span class="token operator">:</span> foo
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> obj2 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

obj1<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span> <span class="token number">2</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> obj1<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>

obj1<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span> obj2<span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> obj2<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>

<span class="token keyword">var</span> bar <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">obj1<span class="token punctuation">.</span>foo</span><span class="token punctuation">(</span> <span class="token number">4</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> obj1<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> bar<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 4</span>
</code></pre></div><p>可以看到，new绑定的优先级<code>&gt;</code>隐式绑定</p> <h3 id="new绑定-vs-显式绑定"><a href="#new绑定-vs-显式绑定" class="header-anchor">#</a> <code>new</code>绑定 VS 显式绑定</h3> <p>因为<code>new</code>和<code>apply、call</code>无法一起使用，但硬绑定也是显式绑定的一种，可以替换测试</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">something</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> something<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> obj1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> bar <span class="token operator">=</span> <span class="token function">foo</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span> obj1 <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">bar</span><span class="token punctuation">(</span> <span class="token number">2</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> obj1<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>

<span class="token keyword">var</span> baz <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">bar</span><span class="token punctuation">(</span> <span class="token number">3</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> obj1<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> baz<span class="token punctuation">.</span>a <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
</code></pre></div><p><code>bar</code>被绑定到obj1上，但是<code>new bar(3)</code> 并没有像我们预计的那样把<code>obj1.a</code>修改为3。但是，<code>new</code>修改了绑定调用<code>bar()</code>中的<code>this</code></p> <p>我们可认为<code>new</code>绑定优先级<code>&gt;</code>显式绑定</p> <p>综上，new绑定优先级 &gt; 显示绑定优先级 &gt; 隐式绑定优先级 &gt; 默认绑定优先级</p> <h2 id="相关链接"><a href="#相关链接" class="header-anchor">#</a> 相关链接</h2> <ul><li><a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/this" target="_blank" rel="noopener noreferrer">this - MDN<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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